1. Field of the Invention
This invention relates generally to systems for monitoring the quality of a weld bead, and more particularly to systems capable of monitoring the speed at which the weld bead is produced by a welding apparatus.
2. Discussion of the Related Art
During the welding process, changes in various welding parameters, such as the travel speed of the welding bead, can occur without the knowledge of the operator. Changes in travel speed can directly result in weld defects, as well as cause changes in weld heat input, which can produce additional weld defects.
One travel speed related defect in a weld is an undercut: a groove melted into the base plate at the toe of the weld, which is caused primarily by a travel speed that is too fast with respect to the welding current employed.
Another travel speed related defect is a slag inclusion: entrapment of an oxide or other non-metallic material under the weld bead. A major source of slag is the coating on stick electrodes, and the amount of slag inclusion is related to the heat input to the weld.
Incomplete fusion--the failure of adjacent layers of the weld metal or weld baseplate to fuse--is another travel speed related defect. Incomplete fusion may result when adjacent metal is not heated to the melting point because of insufficient heat input to the weld.
The cost of locating and repairing weld defects can be a major construction cost. For example, weld inspection can constitute 25-40% of weld fabrication costs. Consequently, it would be desirable to have a real-time weld quality monitor system and method which could monitor selected welding parameters and provide an alarm or cause interruption in welding when defects are detected, thereby eliminating costly rework of defective welds.
Further, the cooling cycle after a weld pass determines the microstructure of the weld metal in the heat-affected zone. With fast cooling rates, some steels become very hard because of a martensitic transformation. If cooling is sufficiently slow, the metal may be more ductile and the structure ferritic and pearlitic. The type of steel generally determines which of these structures is desired. For example, the pearlitic structure is desirable for low carbon and low alloy steels, whereas the martensitic structure is preferred for high strength quenched and tempered steel.
Accordingly, control of the cooling rate is essential to prevent undesirable microstructure in the weld and heat effected baseplates. This control of the weld cooling rate can be obtained through control of the heat input to the weld.
Systems and methods have been suggested for monitoring and controlling certain welding parameters.
For example, U.S. Pat. No. 3,236,997 to Johnson et al. shows a system for controlling the length of a welding arc. A photosensitive transducer, such as a photodiode, is positioned to sense the illumination emitted by the arc of an arc welder between the electrode and the workpiece. A sensing system includes a tubular light pipe and a mirrored surface to project a beam of detectible illumination from the arc through the light pipe to the photodiode sensor.
U.S. Pat. No. 3,262,006 to Sciaky et al. shows a welding control system including a light sensitive probe which incorporates an arrangement using fiber optics to transmit the illumination from the area adjacent the weld to a sensor or phototransducer which is disposed at a remote location.
U.S. Pat. No. 3,370,151 to Normando shows a control system using radiant energy detector scanning. The Normando apparatus generates a train of electrical pulses indicative of successive temperature profiles along lines transverse to a weld bead at a point at a fixed distance from the welding electrode. The Normando apparatus uses a detector which is sensitive mainly to infrared radiation and to a mixture of infrared and visible red radiation. The output of the Normando system can be used for any of various purposes, for example, to vary a selected parameter of the welding machines such as speed of electrode travel along the seam, or to control the welding current.
Other patents which may be considered to be of interest include: U.S. Pat. No. 3,497,668 to Hirsch; U.S. Pat. No. 3,567,889 to Iceland et al.; U.S. Pat. No. 3,602,687 to Pollock; U.S. Pat. No. 3,838,727 to Levi et al.; and U.S. Pat. No. 4,093,844 to Fellure et al.